The proposed study will elucidate the effects of ambient photoperiod on two physiological functions, basal metabolic rate and thyroid function. In addition, the interaction between ambient temperature and photoperiod will be examined, and the data will contribute to an integrated model for the mechanics of thyroid function shifts. Specifically, winter- and summer-caught animals will be measureD for BMR and the utilization rates of thyroxine (T4) and triiodothyronine (T3). Then additional animals will be placed in 10 experimental treatments of 8, 10, 12, 14, and 16 hours of light/day, and either 5 C or 30 C. After eight weeks, these animals will also be measured for BMR and thyroid hormone utilization to ascertain what role photoperiod plays in the determination of "seasonal" levels of BMR and thyroid function. The results will be analysed for nonlinear effects and nonlinear interactions using sophistocated state-of-the-art analyses. This study will examine, in detail, one of the many physiological effects of a changing environmental photoperiod. A thorough analysis of these effects is a prerequisite for 1) understanding such maladies as "winter" depression" (seasonal affective disorder) and "jet lag", 2) improving human performance at work and at play, and 3) efficacious use of phototherapy and related treatments.